An area approach to forest slope stability

Abstract:

An area approach to analyzing the slope stability of thinly-soiled forest areas under consideration for timber harvest is presented. The objective of the research was to develop a probabilistic approach that considers the effects of subsurface drainage concentration, the decay and regrowth of tree roots, the variable nature of soil, and the stochastic nature of precipitation events which typically trigger landslides. The analysis is useful in comparing the stability of an area under alternative land use scenarios.To perform the analysis, the area to be analyzed is divided into a rectangular grid, composed of square grid cells, each with an associated soil depth and elevation. A topographic analysis determines the slope, aspect, and upslope contributing area for each cell. A hydrologic analysis routes the largest 24 hour precipitation event of each year to solve for the peak annual water table conditions at each cell. The root reinforcement and the vegetation loading are modeled through time using empirical relationships describing the decay and regrowth of trees and tree roots. The factor of safety is solved at each grid cell for every year of every trial using a Monte Carlo analysis of the infinite slope equation to account for the uncertainty of the most important variables.By having both spatial and temporal dimensions in the model, the probabilistic analysis is able to account for uncertainty and spatial variation of the input parameters, and the stochastic nature of weather. In addition, the spatial dimension provides a more meaningful method of combining the specific conditions that exist at the individual grid cells (as opposed to totally random combination). For example, the specific soil depth that occurs at a point, will be combined with the specific slope and water table conditions that also exist at that point.